Flexible graphite composite

ABSTRACT

Flexible graphite sheet having embedded ceramic fibers extending from its surfaces into the sheet to increase the permeability of the sheet to resin.

This is a divisional application of Ser. No. 08/833,136, filed Apr. 4,1997.

FIELD OF THE INVENTION

The present invention relates to a composite of flexible graphite andneedle-shaped ceramic particles which can be used in the making ofgaskets.

BACKGROUND OF THE INVENTION

The term "flexible graphite" as used herein represents the exfoliatedreaction product of rapidly heated natural graphite particles which havebeen treated with an agent that intercalates into the crystal-structureof the graphite to expand the intercalated particles at least 80 or moretimes in the direction perpendicular to the carbon layers in the crystalstructure. Flexible graphite and the making thereof is described in U.S.Pat. No. 3,404,061 Shane et al. Expanded, i.e. exfoliated graphite maybe compressed into thin sheets (hereinafter referred as flexiblegraphite "foil") with a density approaching theoretical density,although a density of about 50 to 85 lbs./ft.³ is suitable for mostapplications, including compression into shapes suitable as seal ringsin engine exhaust and other applications.

A common method for making expandable graphite particles, described byShane et al in U.S. Pat. No. 3,404,061 the disclosure of which isincorporated herein by reference. In the typical practice of thismethod, natural graphite flakes are intercalated by dispersing theflakes in a solution containing an oxidizing agent e.g., a mixture ofnitric and sulfuric acid. The intercalation solution contains oxidizingand other intercalating agents known in the art. Examples include thosecontaining oxidizing agents and oxidizing mixtures, such as solutionscontaining nitric acid, potassium chlorate, chromic acid, potassiumpermanganate, potassium chromate, potassium dichromate, perchloric acid,and the like, or mixtures, such as for example, concentrated nitric acidand chlorate, chromic acid and phosphoric acid, sulfuric acid and nitricacid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid,and a strong oxidizing agent soluble in the organic acid. A preferredintercalating agent is a solution of a mixture of sulfuric acid, orsulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitricacid, perchloric-acid, chromic acid, potassium permanganate, hydrogenperoxide, iodic or periodic acids, or the like. Although less preferred,the intercalation solutions may contain metal halides such as ferricchloride, and ferric chloride mixed with sulfuric acid, or a halide,such as bromine as a solution of bromine and sulfuric acid or bromine inan organic solvent. After the flakes are intercalated excess solution isdrained from the flakes and after washing with water, the intercalatedgraphite flakes are dried and are expandable upon exposure to a flamefor only a few seconds. The thus treated particles of graphite arehereafter referred to as "particles of intercalated graphite". Uponexposure to high temperature, the particles of intercalated graphiteexpand in dimension as much as 80 to 1000 or more times its originalvolume in an accordion-like fashion in the c-direction, i.e. in thedirection perpendicular to the crystalline planes of the constituentgraphite particles. The exfoliated graphite particles are vermiform inappearance, and are therefore commonly referred to as worms. The wormsmay be compressed together into flexible sheets which, unlike theoriginal graphite flakes, can be formed and cut into various shapes.

Flexible graphite foil is coherent, with good handling strength, andflexible graphite foil can be wound into rolls and can also be wrappedaround metal fixtures such as mandrels, possesses desirable heattransfer properties and is, accordingly, particularly useful for hightemperature applications such as engine exhaust seal ring applications.It has been proposed to increase the sealability of flexible graphitesheet or foil by impregnating the same with resin. However, the surfacelayers of flexible graphite sheet or foil, due to the alignment ofexfoliated graphite particles and the constituent layers of atomsparallel to the surface of the flexible sheet or foil, resist resinimpregnation when the sheet or foil is immersed in liquid resin.However, due to the well known anisotropy of flexible graphite, resinwill flow readily within the flexible graphite sheet in directionsparallel to the opposed parallel planar surfaces of the sheet or foiland the planes of the constituent graphite particles of the sheet, i.e.transverse to the "c axis" direction of the graphite particles ifpenetration into the flexible graphite sheet can first be achieved.

Accordingly, it is an object of the present invention to provide aflexible graphite sheet, or foil, of increased permeability to enableincreased resin impregnation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows, in an enlarged cross-section sketch of a sheet of flexiblegraphite (original thickness of 0.01 inch) containing ceramic fibers inaccordance with the present invention;

FIGS. 2 through 6 are electron microscope views (original magnification100X) at increasing electron beam intensity voltages (2.5 KV to 40 KV)of a portion of a planar surface of a flexible graphite sheet containingceramic fibers in accordance with the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a composite of a sheet of flexiblegraphite having opposed parallel, planar outer surfaces andneedle-shaped ceramic fiber particles which are embedded in the flexiblesheet and which extend from interior the flexible sheet to at least oneof the planar outer surfaces of the flexible graphite sheet.

DETAILED DESCRIPTION

In the practice of a particular embodiment of the present invention,intercalated natural graphite flakes are mixed and blended with fromabout 1.5 to 30% by weight of needle-shaped ceramic fiber particleshaving a length of 0.15 to 1.5 millimeters. The width of the particlesshould be from 0.04 to 0.004 mm. The ceramic fiber particles arenon-reactive and non-adhering to graphite and are stable at temperaturesup to 2000° F., preferably 2500° F. Suitable ceramic fiber particles areformed of macerated quartz glass fibers, carbon and graphite fibers,zirconia, boron nitride, silicon carbide and magnesia fibers, naturallyoccurring mineral fibers such as calcium metasilicate fibers, calciumaluminum silicate fibers, aluminum oxide fibers and the like.

The mixture of intercalated natural graphite flake and needle-shapedceramic fiber particles, in which the graphite flake and ceramic fiberparticles are in general alignment, is exposed to a flame at atemperature up to 2500° F. to exfoliate, i.e. expand the intercalatedgraphite flake into expanded graphite particles from 80 to 1000 timesthe volume of the unexpanded intercalated natural graphite flake whichsurround and entrap the needle-shaped ceramic fiber particles. Due tothe expansion, the ceramic fiber particles are no longer in substantialalignment with the graphite particles but are randomly positioned in themixture of exfoliated graphite and ceramic fiber. The mixture ofexfoliated graphite particles and randomly oriented needle-shapedceramic fiber particles is roll-pressed into sheet or foil typically 0.1to 3.5 mm thick. The resultant sheet, or foil, is characterized byhaving needle-shaped particles extending from interior of the flexiblegraphite sheet to and through at least one of the opposed planarsurfaces of the flexible graphite sheet. Since the needle-shaped ceramicfiber particles are non-reactive and non-adhering to the graphite in theflexible sheet, a plurality of annular channels surrounding therespective needle-shaped particles is provided in the flexible graphitesheet extending from the opposed surfaces of the sheet into the body ofthe sheet. These channels, with the flexible graphite sheet immersed inliquid resin, receive the resin which then infiltrates the flexiblegraphite sheet in the more permeable direction parallel to the planarsurfaces of the flexible graphite sheet and the pressed, exfoliatedgraphite particles forming the sheet, assisted by channels formed byneedle shaped ceramic fiber particles embedded between, but notpenetrating the planar parallel surfaces of the flexible graphite sheet.The ceramic fiber particles remain stable during all steps of theprocessing treatment so that the channels are not blocked by meltedfiber or fiber decomposition products. After curing of the resin, withinthe graphite sheet the sealability of the flexible graphite sheet isenhanced for gaskets formed from said sheet. In a preferred embodimentthe resin-containing sheet is calendered by roll pressing betweenpressure rolls.

FIG. 1 is a drawing sketch based on microscope observations of 0.01 inchthick sheets of flexible graphite which shows a flexible graphite sheet10, in cross-section, with parallel opposed planar surfaces 22, 24.Embedded ceramic fiber particles are shown at 30. Penetration of sheet10 by ceramic fibers 30 is illustrated at 40.

EXAMPLE I

Natural graphite flake, sized 80% by weight held on 50 mesh was treatedin a mixture of 90 wt. % sulfuric acid and 10 wt. % nitric acid. Thethus treated intercalated graphite flake was water washed and dried toabout 1% by weight water. One (1) pound of this intercalated flake wasmixed with 0.15 pounds of needle-shaped ceramic fiber of commerciallyavailable calcium metasilicate sized mostly with a 15 to 25% aspectratio. The mixture of intercalated graphite and calcium metasilicatefiber was introduced into a furnace of 2500° F. to obtain rapidexpansion of the intercalated graphite flake into vermicular worm-shapedparticles having a volume of about 325 times that of the unexpandedintercalated flake. The expanded worm-shaped particles surrounded theadmixed calcium metasilicate fibers and the mixture was rolled into aflexible graphite sheet 0.01 inch thick and 24 inches wide in whichadmixed calcium metasilicate fibers extended from a surface of the sheetinto the body of the sheet which contained about 12% by weight of thecalcium metasilicate fiber.

The electron microscope view of FIG. 2 (100X- see 100 micron scale)shows the upper portion of a ceramic fiber 100 which penetrates a planarsurface 22 of a flexible graphite sheet. FIGS. 3 to 6, at increasingvoltages, look "deeper" into the flexible graphite sheet and show thepenetration of ceramic fiber 100 into the flexible graphite sheet.Ceramic fibers embedded in the flexible graphite sheet below surface 72are shown at 140, 160.

EXAMPLE II

A sample of the sheet of Example I, 8 inches wide was immersed in andpulled through a solution of resin 10% phenolic resin with acetonediluent at a rate of 10 feet per minute. After immersion and drying, thesample showed an increase in weight of 18.7%.

The sample was further treated by heating to 235° C. to stabilize theresin and the sheet was calendered between pressure rolls to a densityof 1.5 gms/cc. The calendered sheet was soaked in oil and water and wasunaffected. A control sheet without any added ceramic fiber or resinadditives exposed to the same test conditions increased about 35% inweight and 8% in thickness.

Sample sheets with 5%, 15% and 25% by weight of calcium metasilicatefibers as additive were pulled through resin at a rate of 10 feet perminute and were saturated with resin at about 17-19% by weight of resin.A control sample without any added ceramic fiber retained only 5% byweight of resin at the same pull rate of 10 feet per minute.

What is claimed is:
 1. Method for making a flexible graphite sheethaving opposite planar outer surfaces which is permeable to liquid resinsolution which comprises:(i) treating natural graphite flake with anintercalating solution to obtain heat expandable, intercalated graphiteflake particles; (ii) mixing from about 1.5 to 30% by weight ofneedle-shaped ceramic fiber particles with said intercalated graphiteflake particles to provide a blended mixture, said needle-shaped ceramicfiber particles being non-reactive with flexible graphite and stable attemperatures up to 2500° F.; (iii) passing said blended mixture ofintercalated natural graphite flake particles and needle-shaped ceramicfiber particles through a flame to cause rapid exfoliation of saidintercalated natural graphite particles and provide a mixture of saidneedle-shaped ceramic fiber particles with exfoliated graphiteparticles; and (iv) passing said mixture of needle-shaped particles andexfoliated graphite particles between pressure rolls to form acompressed sheet of flexible graphite having embedded therein adispersion of a plurality of needle-shaped particles which extend fromat least one of said planar outer surfaces into the sheet.
 2. Method inaccordance with claim 1 wherein (i) the sheet of step (iv) is immersedin resin which penetrates the sheet; (ii) stabilizing the resin byheating; and (iii) calendering the resin containing sheet by passing theresin containing sheet between pressure rolls.